console: Introduce console_start() and console_stop()

console: Introduce console_start() and console_stop()

In the future we would like to stop unused consoles and
also add a reference counting to avoid imbalanced calls
to ->start() and ->stop() in some cases.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>